1. Field of the Invention
The present invention relates to a tensioner and in particular to a tensioner for tensioning a belt, a chain or the like.
2. Description of the Related Art
A tensioner for preventing slackening of a belt, a chain or the like has been disclosed in U.S. Pat. No. 4,539,001, for example. The conventional tensioner, as shown in FIG. 4, includes a body 51 having therein a large diameter chamber 54 and a plunger 52 which is slidably mounted in the body 51. The plunger 52 is urged continually towards the large diameter chamber 54 by a spring 57. In the plunger 52, there is formed a bore 58 in which a rod 59 is slidably fitted. Between the plunger 52 and the rod 59 is defined a small diameter chamber 61. The small diameter chamber 61 is in fluid communication with the large diameter chamber 54 through a passage 60 formed at a bottom wall of the bore 58. Operating fluid is filled in both chambers 54 and 61 and in the passage 60. The rod 59 extends from the body 51 so as to be engaged with a belt (not shown) to be tensioned. For assuring the fluid-tightness between the plunger 52 and the body 51 and the fluid-tightness between the plunger 52 and the rod 59, a seal member 53 is provided on the plunger 52 and a seal member 62 is provided on the rod 51, respectively.
With such a construction, when the tension of the belt is increased as a result of an increase in ambient temperature around the belt, the load corresponding to the increment in the belt tension is applied to the rod 59. Due to the resulting load increment, the rod 59 is moved downwardly at a distance so as to absorb the increase in the belt tension. During downward movement of the rod 59, fluid in the small diameter chamber 61 is supplied to the large diameter chamber 54 through the passage 60, thereby moving the plunger 52 in the upward direction against the load of the spring 57. On the other hand, when the tension is decreased, the rod 59 is moved upwardly due to the inflow of the fluid into the small diameter chamber 61 from the large diameter chamber 54 as a result of the expansion of the spring 57.
However, since a high pressure is generated in both chambers 61 and 51 upon downward movement of the rod 59, repeating applications of the high pressure on the seal members 53 and 62 cause damage or deformation thereof with the result that the fluid-tightness between the body 51 and the plunger 52 and the fluid-tightness between the plunger 52 and the rod 59 are not assured. In addition, due to such deformation of the seal members, air is able to invade the fluid, thereby causing erratic operation or the malfunction of the tensioner.